Immune tolerance may decrease the therapeutic benefit of immune based therapies. Therefore, understanding the mechanisms that cause tumor-induced tolerance could significantly improve the success of cancer immunotherapy. Dysfunctional T cells of cancer patients develop unique molecular changes including a decreased CD3zeta chain and Jak-3 tyrosine kinase and an inability to translocate NFkappaBp65. Recently we demonstrated that T cells stimulated in the absence of L-arginine develop the same molecular and functional alterations, suggesting that the depletion of this amino-acid could help explain T cell tolerance in cancer. We are studying the molecular alterations induced by the arginine depletion in T cells. However, the mechanisms regulating arginine availability in cancer patients are unclear. Our preliminary data demonstrate that arginase I is produced at high levels in the tumor microenvironment by mature myeloid cells and not by tumor cells, immature myeloid cells or regulatory T cells. Soluble factors produced by tumor cells and infiltrating T cells appear to regulate arginase I in tumor associated mature myeloid cells. Furthermore, blocking arginase I and replenishing arginine in tumor bearing mice causes a significant antitumor effect. Thus, our hypothesis states that "Arginase produced in the tumor microenvironment depletes L-Arginine, which induces molecular and functional alterations in T cells and impairs the development of a protective anti-tumor response. Blocking arginase or replenishing L-Arginine will re-establish T cell activity and a therapeutic anti-tumor response". The overall goal of this proposal is to understand how arginase I is regulated in cancer. In vitro models will be used to understand the molecular regulation of arginase I by prostanoids and cytokines produced in the tumor microenvironment. Transgenic and knock-out strains of mice and tumor cells will determine how arginase is regulated in tumor bearing mice. We will then test the significance of this mechanism of tumor induced tolerance by studying it in patients with lung and colon carcinoma.